The application of electrochemical measurements to the study and behaviour of zinc-rich coatings

Abstract

Three zinc-rich coatings, two with polyamide cured epoxy binders and one with an ethyl silicate binder applied to grit blasted mild steel were chosen for examination. The two epoxy coatings differed in terms of their particle size distribution, but were similar in all other respects. The silicate system was chosen to be very similar to one of the epoxy systems except for the chemical nature of the binder. Techniques chosen to study these coatings included impedance, potential vs time for a fully coated and partially coated system, exterior exposure under marine conditions and SEM on film sections during exposure. The electrochemical response of each system was found to be highly dependent on the particle size distribution of the zinc rich pigment. Where pigment particles were comparable in size with the film thickness, the electrochemical behaviour could be modelled in terms of simple charge transfer processes. With smaller zinc particles, a diffuse time constant model is more appropriate. In the silicate system, a porous silicate layer is formed above the zinc-rich layer and the electrochemical response is determined by the nature of this porous layer.